Can't find the specs on the 120, but the 125 is the same as the 1.00 with a bigger bore. I expect the 1.20 was a bigger block, on the same framework as the 1.50 and 1.80. Seems that Saito si going more and more to oversquare designs, where the stroke for a given bore is shorter. The 1.25 is lighter and has a bit more power, and probably revs quicker, than the 1.20.

Looking in the Saito brochure, many of the 1.25's outer dimensions are actually SMALLER than the 1.00, such as the distance from rear of the case to the tip of the prop shaft, and from the bottom of the case to the top of the valve covers. 1.00 makes 1.8 hp, the 1.25 makes 2.2 hp.

One of the neat things about the 1.20, don't know if the others (1.25, 1.50 and up) do it, is you can exchange the intake and exhaust (put the carb on the exhaust side, run the pipe out the intake), and reverse the engine rotation. The valve timing was symmetrical, so no changes necessary to valve timing.